Insulating a Plutonian Ocean

byPaul GilsteronMay 22, 2019

An ocean inside Pluto would have implications for many frozen moons and dwarf planets, not to mention exoplanets where conditions at the surface are, like Pluto, inimical to life as we know it. But while a Plutonian ocean has received considerable study (see, for example, Francis Nimmo’s work as discussed in Pluto: Sputnik Planitia Gives Credence to Possible Ocean), working out the mechanisms for liquid ocean survival over these timeframes and conditions has proven challenging. A new paper now suggests a possible path.

Shunichi Kamata of Hokkaido University led the research, which includes contributions from the Tokyo Institute of Technology, Tokushima University, Osaka University, Kobe University, and the University of California, Santa Cruz. At play are computer simulations, reported in Nature Geosciences, that offer evidence for the potential role of gas hydrates (gas clathrates) in keeping a subsurface ocean from freezing. At the center of the work, as in so much recently written about Pluto, is the ellipsoidal basin that is now known as Sputnik Planitia.

Image: The bright “heart” on Pluto is located near the equator. Its left half is a big basin dubbed Sputnik Planitia. Credit: Figures created using images by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Gas hydrates are crystalline solids, formed of gas trapped within molecular water ‘cages’. The ‘host’ molecule is water, the ‘guest’ molecule a gas or a liquid. The lattice-like frame would collapse into a conventional ice crystal structure without the support of the trapped molecules.

What is illuminated by the team’s computer simulations is that such hydrates are highly viscous and offer low thermal conductivity. They could serve, in other words, as efficient insulation for an ocean beneath the ice. The location and topography of Sputnik Planitia lead the researchers to believe that if a subsurface ocean exists, the ice shell in this area is thin. Uneven on its inner surface, the shell could thus cover liquid water kept viable by the insulating gas hydrates.

We wind up with a concept for Pluto’s interior that looks like the image below.

Image: The proposed interior structure of Pluto. A thin clathrate (gas) hydrate layer works as a thermal insulator between the subsurface ocean and the ice shell, keeping the ocean from freezing. Credit: Kamata S. et al., “Pluto’s ocean is capped and insulated by gas hydrates.” Nature Geosciences, May 20, 2019 (full citation below).

Methane is implicated as the most likely gas to serve within this insulating lattice, a provocative theory because of what we know about Pluto’s atmosphere, which is poor in methane but rich in nitrogen. What exactly is happening to support this composition? From the paper:

CO2 clathrate hydrates at the seafloor could have acted as a thermostat to prevent heat transfer from the core to the ocean. Primordial CO2, however, may have been converted into CH4 through hydrothermal reactions within early Pluto under the presence of Fe–Ni metals. As CH4 and CO predominantly occupy clathrate hydrates, the components that degassed into the surface–atmosphere system would be rich in other species, such as N2.

We arrive at an atmosphere laden with nitrogen but low in methane. This notion of a thin gas hydrate layer as a ‘cap’ on a subsurface ocean is one that could serve as a generic mechanism to preserve subsurface oceans in large, icy moons and KBOs. “This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” adds Kamata.

The authors point out that freezing of the ocean, causing the ice shell to thicken, would cause the radius and surface area of Pluto to increase, producing faults on the surface. New Horizons was able to observe these, and recent studies have shown that the fault pattern supports the global expansion of the dwarf planet. Thus we have a scenario consistent with a global ocean, perhaps one that is still partly liquid. Analyzing surface changes would offer constraints on the thickness of any potential layers of clathrates that could firm up the liquid water hypothesis.

While speculative, it is an interesting idea. The author’s state that this mechanism might allow for a subsurface ocean on Titan, as well as the icy moons too.

If such methane clathrates formed in Europa, they would insulate the ocean from the surface, allowing a slightly warmer ocean as long as it was below the melting points of the clathrates.

I wonder whether methane clathrates periodically melting and releasing some of the methane as a gas might provide the extra pressure needed to create a plume, rather like a soda siphon. The cycle would start with the crust conducting away the heat from its base, causing the clathrates to form from methane released from the core. This insulates the crust, allowing the ocean to warm. Then the clathrates melt, some of the gas is released from solution, creates plumes in weak spots in the crust, restarting the cycle.

What I find most interesting in the author’s hypothesis, is that this mechanism could provide a useful thermostat that maintains conditions in response to varying heat fluxes. The author’s use it to show how an ocean could be maintained over 4+ gy rather than slowly freezing solid. But it might also provide a way to maintain ocean stability if internal or external conditions change.

Wouldn’t the CO2 clathrate shielding the core from the ocean stop any cycling of nutrients, etc between the core and the ocean? If that is true, it makes it more difficult for life arising or surviving. This wouldn’t enhance the chances for ET.

In this model, the CH4 clathrate is between the ocean and the ice crust, not on the ocean floor. Therefore I am not clear that your comment is relevant. However, CO2 clathrates would sit on the ocean floor. This is part of a model for waterworlds with dense CO2 atmospheres proposed by Dr. Ramirez Rameses for their habitability. I see no reason why such CO2 clathrates should not form on Pluto’s hypothesized ocean bottom, sealing off the core from the ocean. At first glance, with little core heat, there would seem to be little to disrupt such a layer.

It’s curious that when speculating about subsurface liquids on alien worlds such as Pluto that we automatically assume that liquid is mostly or entirely water. I must point out that besides nitrogen, water and methane, the other major constituent of Sputnik Planitia, carbon monoxide, is at its triple point just beneath the surface (−205.05 °C, 0.1517 atm). Compounds at their triple point are able to support a host of complex chemistries, any of which could explain the anomalous readings seen by New Horizons. Add to that carbon monoxide is a polar molecule, making it a potent solvent.

I suspect CO is not considered because it is a minor component, compared to water. That is not to rule out CO as a component, but at 15% at most of cometary bodies, it is unlikely to be the basis for an ocean, rather than a lake.

Nitrogen is found in abundance at Sputnik Planitia and its triple point is lower than CO. N2 triple point is 63 K or -210 C. The abundance of N2 suggests it has oozed up from below. In essence Sputnik Planitia appears to be like a caldera with N2 as its lava.

And the source of the molecular N2 is? Ammonia? Something else? The author’s model aids this effect, I think, by allow the crust to be colder than if it was not insulated. But that is purely speculation on my part. What is more important is how molecular nitrogen is formed.

Back when it was not certain there would ever be a probe mission to Pluto, astronomers attempted to console themselves after the Voyager 2 mission to Neptune by saying that Triton was their Pluto substitute.

This structure looks a bit like a Gulf Coast salt dome-a good container for a liquid and or gas reservoir isolated from the surface and sides. I’m not sure how enough energy could be generated or maintained for an exotic life form, along with cycling of nutrients and waste products.

Of course this is all speculative until a Plutonian or Tritonian rover sticks a pin or explosive harpoon into a suspected structure and samples the escaping gasses and liquids, should they exist.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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